1. Field of the Invention
The present invention relates to a signal generation device and a signal generation method that impart characteristics of second-order cyclostationarity in a communication system that carries out signal identification based on second-order cyclostationarity.
2. Related Background Art
In recent communication systems, communication has come to be carried out by recognizing communication status by gathering information on signals received by a terminal that has received a signal, analyzing the recognized communication status, and using signal transmission parameters that enable the attaining of communication quality (prescribed quality of a communication rate or error rate and the like) that is convenient for or desired by the terminal based on the results of that analysis. Moreover, studies have been conducted on environment recognition communication systems that carry out such communication particularly in the field of wireless communication. In addition, in environment recognition communication systems, gathering of information based on signal demodulation is considered to be typical for recognizing communication status at a terminal.
However, in the case of carrying out communication within the same area in which a plurality of systems is present having different communication methods, since signals cannot be mutually demodulated, although it is possible to recognize the state of “a signal being present” when a signal has been received, there is the problem of being unable to gather any information beyond that.
In contrast, techniques have been attracting attention that focus on signal statistics and gather information on received signals by calculating statistics. Among these techniques, studies have been conducted in particular on second-order cyclostationarity, which has fewer arithmetic operations. Second-order cyclostationarity refers to statistics that generate different characteristics for signals having different parameters, and the use thereof makes it possible to easily identify a plurality of signals having different communication methods. Thus, in the case of carrying out communication within the same area where a plurality of systems having different communication methods is present, in addition to information in the form of “a signal is present”, information as to “which signal belonging to which system” can also be obtained when a signal has been received. However, even if this technique is used, there was still the problem of the amount of information obtained being extremely small and being inadequate for recognizing communication status.
Therefore, a technique has been studied that enables a transmitter to impart characteristics of second-order cyclostationarity to a signal to be generated, and to transmit a larger amount of information by means of artificially imparted statistics. Examples of the related art are U.S. Patent Publication No. 2008-0026704 (to be referred to as Reference 1) and P. D. Sutton, K. E. Nolan and L. E. Doyle, “Cyclostationary Signatures in Practical Cognitive Radio Applications”, IEEE Journal on Selected Areas in Communications (JSAC), Vol. 26, no. 1, pp. 13-24, 2008 (hereinafter, referred to as Reference 2).
Reference 1 describes a signal generation method in which an ID based on statistics is imparted to a radio signal. Paragraphs 64 to 73 of the specification of Reference 1 describe a radio signal generation method which imparts characteristics of cyclostationarity in a multi-carrier transmission system by transmitting the same symbol for a plurality of predetermined subcarriers. Reference 2 describes a radio signal generation method which imparts characteristics of cyclostationarity by duplicating data transmitted by some of subcarriers at the time of the use of an OFDM (Orthogonal Frequency Division Multiplexing) system and controlling input to inverse Fourier transform to generate transmission signals such that the same data is transmitted by other subcarriers.
The radio signal generation method in Reference 1 and the radio signal generation method in Reference 2 make the transmission signal of a certain carrier identical to a signal transmitted by a subcarrier distant from the relevant subcarrier by a predetermined number of subcarriers, thereby generating signals having characteristics of cyclostationarity corresponding to the predetermined number. However, according to these inventions, since the types of characteristics of cyclostationarity which can be imparted is limited by the total number of subcarriers, there was a problem in that it is difficult to transmit numerous types of information using characteristics of cyclostationarity.